1. Field of the Invention
The present invention applies to the art of mass spectrometry, and in particular to interpreting mass spectra with multiply charged ions in the presence of noise, mixtures and contaminants, especially for low charge, low molecular weight analytes such as peptides.
2. Art Background
The mass spectrometer produces information on mass-to-charge ratios, often shown as m/z, of analytes in a sample. This information must be interpreted to assign molecular weights to the analyte, the sample being analyzed. The process used in electrospray ionization produces ions with multiple charge states, that is, ions at different m/z values where the mass m is the same for each ion, but the charge z is different. Interpreting these spectra with multiply charged ions involves the process of deconvoluting the ions to obtain a molecular weight assignment of the uncharged analyte. In the real world, this process must be accomplished in the presence of instrument noise, contaminants, mixtures, and artifacts.
For high molecular weight analytes, the analysis is relatively simple: 1) an ion is chosen, 2) an arbitrary charge state is assigned to the ion, 3) surrounding ions are tested to see if a charge series exists, and 4) the process is repeated until all related ions in the series are found. This process works reasonably well for high molecular weight (&gt;8000 Daltons) analytes because 1) at high molecular weights the isotope distribution becomes Gaussian, 2) the series test can require many charge states to succeed since many charge states will always be present, and 3) most mass analyzers do not have the resolving power to separate the isotopes of a highly charged ion. An approach similar to this is described in U.S. Pat. No. 5,130,538 to Fenn, et. al.
In comparison, deconvoluting low charge, low molecular weight analytes such as peptides is more problematic: 1) often only one or two charge states exist, thus making series testing more difficult or impossible to perform, 2) isotope patterns do not follow a Gaussian distribution for low molecular weights, and 3) isotopes can be resolved for low charge states.
The process disclosed by Fenn cannot correctly interpret these low charge, low molecular weight analytes. The comparatively large number of charge states required by Fenn are not present, and extending the process disclosed in Fenn to work with a low number of charge states often results in errors of assignment. Because only one or two charge states are present, extending Fenn leads to erroneous associations between "noise" ions and signal ions, producing erroneous and misleading results.
What is needed is a way to deal with multiply charged spectra of low charge, low molecular weight analytes in performing molecular weight assignment.